Regiospecific Synthesis and Structural Studies of 3,5-Dihydro‑4H‑pyrido[2,3‑b][1,4]diazepin-4-ones and Comparison with 1,3-Dihydro‑2H‑benzo[b][1,4]diazepin-2-ones

Nine 3,5-dihydro-4H-pyrido­[2,3-b]­[1,4]­diazepin-4-ones (17–25), some of which contain fluoro-substituents, have been regiospecifically prepared by reaction of 2,3-diaminopyridines with ethyl aroylacetates. In two cases, open intermediates have been isolated and these are related to the reaction pathway. The X-ray crystal structure of 1-methyl-4-phenyl-3,5-dihydro-4H-pyrido­[2,3-b]­[1,4]­diazepin-4-one (23) has been solved (formula, C15H13N3O; crystal system, monoclinic; space group, C2/c). This is an asymmetric unit constituted by a single nonplanar molecule and its conformational enantiomer due to the presence of the seven-membered diazepin-2-one moiety, which introduces a certain degree of torsion in the adjacent pyridine ring. The 1H, 13C, 15N, and 19F NMR spectra were obtained and the chemical shifts, together with those of the previously published 1,3-dihydro-2H-benzo­[b]­[1,4]­diazepin-2-ones (1–16), i.e., a total of 544 values, were successfully compared with the chemical shifts calculated at the gauge invariant atomic orbital (GIAO)/Becke, three-parameter, Lee–Yang–Parr (B3LYP)/6-311++G­(d,p) level. The seven-membered ring inversion barrier in 5-benzyl-2-phenyl-3,5-dihydro-4H-pyrido­[2,3-b]­[1,4]­diazepin-4-one (25) was determined and, in conjunction with the data from the literature, compared with the B3LYP/6-311++G­(d,p) computed values. This allowed the determination of several structural effects. The rotation about the exocyclic N1–CR bond was also calculated and its dynamic properties were discussed.